how to match the scale and position of background images
I deal with background images by setting them to the same size before starting Blender in an external image processing package like Gimp. In your case, Ixd adjust the right view by enlarging the front view by padding the right view by adding 93 pixels on both sides of the image, so that the right view has the same dimensions as the front view, that is 310 x 506 pixels. This will make it easier to line up the images with the views.